1. Field of the Invention
This invention relates generally to the field of photovoltaic arrays, and more particularly to dynamic frequency and pulse-width modulation of dual-mode switching power controllers used in photovoltaic arrays.
2. Description of the Related Art
Photovoltaic (PV) arrays—more commonly known and referred to as solar arrays—are a linked collection of solar panels, which typically comprise multiple interconnected solar cells. The modularity of solar panels facilitates the configuration of solar (panel) arrays to supply current to a wide variety of different loads. The solar cells convert solar energy into direct current electricity via the photovoltaic effect, in which electrons in the solar cells are transferred between different bands (i.e. from the valence to conduction bands) within the material of the solar cell upon exposure to radiation of sufficient energy, resulting in the buildup of a voltage between two electrodes. The power produced by a single solar panel is rarely sufficient to meet the most common power requirements (e.g. in a home or business setting), which is why the panels are linked together to form an array. Most solar arrays use an inverter to convert the DC power produced by the linked panels into alternating current that can be used to power lights, motors, and other loads.
The various designs proposed and developed for solar arrays typically fall into one of two configurations: a low-voltage configuration (when the required nominal voltage is not that high), and a high-voltage configuration (when a high nominal voltage is required). The first configuration features arrays in which the solar panels are parallel-connected. The second configuration features solar panels first connected in series to obtain the desired high DC voltage, with the individual strings of series-connected panels connected in parallel to allow the system to produce more current. Various problems have been associated with both configurations, with the most prolific array configuration being the high-voltage series-string based configuration. The series-string configuration raises the overall distribution DC-bus voltage level to reduce resistive losses. However, in doing so it increases panel mismatch losses by virtue of the series-string being limited by the weakest panel in the string. In addition, the resultant DC-bus voltage has a significant temperature and load variance that makes inversion from DC to AC more difficult. Consequently, many design efforts have been concentrated on improving the efficiency of the collection of electrical power from the array, by mitigating these non-idealities.
Various designs have been proposed and developed for DC/DC (DC-to-DC) converter systems applied to solar arrays. Most of these designs have concentrated on the implementation of Maximum Power Point Tracking (MPPT), which employs a high efficiency DC/DC converter that presents an optimal electrical load to a solar panel or array, and produces a voltage suitable for the powered load. Oftentimes the DC/DC converters are implemented with a switching regulator in order to provide highly efficient conversion of electrical power by converting voltage and current characteristics. Switching regulators typically employ feedback circuitry to monitor the output voltage and compare it with a reference voltage to maintain the output voltage at a desired level. While typical regulation may be satisfactory in most applications, when operating DC/DC converters with photovoltaic/solar panels, the requirements associated with such arrays present additional problems that typical regulation cannot adequately address.
For example, one type of commonly used switching power converter in PV array designs featuring DC/DC converters is a Buck-Boost switching power converter. Various solutions exist for improving efficiency in Buck-Boost switching power converters. One widely utilized system-architecture is the dual-mode H-bridge style converter. Several innovations relating to the smooth transition between buck and boost modes for this configuration have been utilized and proposed, each with several fundamental limitations, but none addressing the optimization of efficiency over a wide Vin and Vout operating range. Since a minimum pulse-width is required for any switching event, the transition between buck mode and boost mode involves an abrupt event. Prior solutions address this abrupt event by creating a third mode which is equivalent to a much older, and less efficient, technique, where the 4-switches are all simultaneously utilized in a single continuous mode to alternately transfer current through the primary inductor to emulate a buck and boost function.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.